Role of early particle formation in modeling gas-phase chemistry of Fe-doped flames

We report quantitative flame structure measurements in low-pressure (30 mbar) premixed H₂/O₂/Ar flat flames doped with Fe(CO)₅ (0–500 ppm) across equivalence ratios, ϕ, ranging from 0.25 to 1.5. Absolute concentrations of gas-phase Fe and FeO, alongside temperature profiles, are determined using laser-induced fluorescence (LIF) and intracavity laser absorption spectroscopy (ICAS). For the first time, absolute Fe and FeO 2-D resolved concentration measurements are reported and compared to kinetic simulations over a wide parameter range. 2-D CFD simulations with reduced gas-phase chemistry (29 species/81 reactions) overall capture well the temperature profiles but significantly overpredict Fe and FeO. Formation of early nanoparticles in the preheat zone of the flame qualitatively explains the depletion of Fe-containing species inventory in the gas phase. Nevertheless, purely gas-phase iron chemistry model, which does not account for this important channel of Fe-containing gas-phase species depletion, is capable of capturing experimentally observed temperatures. This manifests the previously reported saturation of catalytic recombination cycles and underscores the importance of absolute speciation data for mechanism validation. A better understanding of the processes responsible for reentrance of Fe-species from the particle to the gas phase and the kinetics of catalytic recombination cycle is required, necessitating coupled gas-particle models. Novelty and significance statement Quantitative data on iron-flame interactions remain scarce, particularly for in-situ temperature, species, and particle measurements. We report systematic flame structure measurements in Fe(CO)₅ -doped H₂/O₂/Ar flames spanning very fuel-lean to very fuel-rich conditions and a wide range of precursor concentrations. Quantitative gas temperatures, Fe-atom concentrations, and, for the first time, absolute FeO concentrations are obtained. Earlier studies reported only Fe and relative FeO within limited equivalence ratio ranges; the present dataset substantially strengthens constraints on the still poorly understood iron combustion mechanism. Complementary simulations tightly couple gas-phase iron chemistry with heterogeneous reactions at particle surface, explicitly incorporating particle surface reactions, long treated as a modeling black box. Together, our results provide first insights into how nanoparticle surface chemistry alters Fe-containing intermediates and constitute an essential step toward resolving the mechanistic gap governing reversible interactions between gas-phase growth species, reactive bath and nascent particles that ultimately control scalable multi-element nano-oxide synthesis.